Examples

Description

Impose an additional acceleration on each particle in the group. This
fix is typically used with granular systems to include a “gravity”
term acting on the macroscopic particles. More generally, it can
represent any kind of driving field, e.g. a pressure gradient inducing
a Poiseuille flow in a fluid. Note that this fix operates differently
than the fix addforce command. The addforce fix
adds the same force to each atom, independent of its mass. This
command imparts the same acceleration to each atom (force/mass).

The magnitude of the acceleration is specified in force/mass units.
For granular systems (LJ units) this is typically 1.0. See the
units command for details.

Style chute is typically used for simulations of chute flow where
the specified angle is the chute angle, with flow occurring in the +x
direction. For 3d systems, the tilt is away from the z axis; for 2d
systems, the tilt is away from the y axis.

Style spherical allows an arbitrary 3d direction to be specified for
the acceleration vector. Phi and theta are defined in the usual
spherical coordinates. Thus for acceleration acting in the -z
direction, theta would be 180.0 (or -180.0). Theta = 90.0 and
phi = -90.0 would mean acceleration acts in the -y direction. For
2d systems, phi is ignored and theta is an angle in the xy plane
where theta = 0.0 is the y-axis.

Style vector imposes an acceleration in the vector direction given
by (x,y,z). Only the direction of the vector is important; it’s
length is ignored. For 2d systems, the z component is ignored.

Any of the quantities magnitude, angle, phi, theta, x, y,
z which define the gravitational magnitude and direction, can be
specified as an equal-style variable. If the value is
a variable, it should be specified as v_name, where name is the
variable name. In this case, the variable will be evaluated each
timestep, and its value used to determine the quantity. You should
insure that the variable calculates a result in the appropriate units,
e.g. force/mass or degrees.

Equal-style variables can specify formulas with various mathematical
functions, and include thermo_style command
keywords for the simulation box parameters and timestep and elapsed
time. Thus it is easy to specify a time-dependent gravitational
field.

Styles with a gpu, intel, kk, omp, or opt suffix are
functionally the same as the corresponding style without the suffix.
They have been optimized to run faster, depending on your available
hardware, as discussed in Section 5
of the manual. The accelerated styles take the same arguments and
should produce the same results, except for round-off and precision
issues.

These accelerated styles are part of the GPU, USER-INTEL, KOKKOS,
USER-OMP and OPT packages, respectively. They are only enabled if
LAMMPS was built with those packages. See the Making LAMMPS section for more info.

You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the -suffix command-line switch when you invoke LAMMPS, or you can
use the suffix command in your input script.

See Section 5 of the manual for
more instructions on how to use the accelerated styles effectively.

The fix_modifyenergy option is supported by this
fix to add the gravitational potential energy of the system to the
system’s potential energy as part of thermodynamic output.

The fix_modifyrespa option is supported by this
fix. This allows to set at which level of the r-RESPA
integrator the fix is adding its forces. Default is the outermost level.

This fix computes a global scalar which can be accessed by various
output commands. This scalar is the
gravitational potential energy of the particles in the defined field,
namely mass * (g dot x) for each particles, where x and mass are the
particles position and mass, and g is the gravitational field. The
scalar value calculated by this fix is “extensive”.

No parameter of this fix can be used with the start/stop keywords of
the run command. This fix is not invoked during energy minimization.